If we move the end of the wire from _b_ to _d_ the pointer will take a new position. Opposite this we mark twice the number of volts of the standard cell. We can run it to a point _e_ where the distance _ne_ is one-half _nb_, and mark our scale with half the number of volts of the standard cell, and so on for other positions along the wire. That's the way we calibrate a sensitive current-measuring instrument (with its added wire, of course) so that it will read volts. It is now a voltmeter. If we connect a voltmeter to the battery _X_ as in Fig. 16 the pointer will tell us the number of volts in the e. m. f. of _X_, for the pointer will take the same position as it did when the voltmeter was connected between _n_ and _d_. There is only one thing to watch out for in all this. We must be careful that the voltmeter is so made that it won't offer too easy a path for electrons to follow. We only want to find how hard a battery can pull an electron, for that is what we mean by e. m. f. Of course, we must let a small stream of electrons flow through the voltmeter so as to make the pointer move. That is why voltmeters of this kind are made out of a long piece of fine wire or else have a coil of fine wire in series with the current-measuring part. The fine wire makes a long and narrow path for the electrons and so there can be only a small stream. Usually we describe this condition by saying that a voltmeter has a high resistance. [Illustration: Fig 16] Fine wires offer more resistance to electron streams than do heavy wires of the same length. If a wire is the same diameter all along, the longer the length of it which we use the greater is the resistance which is offered to an electron stream. You will need to know how to describe the resistance of a wire or of any part of an electric circuit. To do so you tell how many "ohms" of resistance it has. The ohm is the unit in which we measure the resistance of a circuit to an electron stream. I can show you what an ohm is if I tell you a simple way to measure a resistance. Suppose you have a wire or coil of wire and want to know its resistance. Connect it in series with a battery and an ammeter as shown in Fig. 17. The same electron stream passes through all parts of this circuit and the ammeter tells us what this stream is in amperes. Now connect a voltmeter to the two ends of the coil as shown in the figure. The voltmeter tells in volts how much e. m. f. is being applied to force the current through the coil. Divide the number of volts by the number of amperes and the quotient (answer) is the number of ohms of resistance in the coil. [Illustration: Fig 17]